The present invention relates to an improved method for extruding plastic tubing with integral profiles for forming reclosable bags, and more particularly to an improved method for forming said tubing of different sizes for bags of different dimensions wherein the interlocking rib and groove profiles on the tubing are the same regardless of the tube size.
This invention relates to tubing for reclosable bags of the type formed of a thin lightweight plastic such as polyethylene having mating interlocking profiles on the inner facing surfaces at the top of the bag which profiles are so shaped that they will interlock when pressed together and will separate when the edges of the bag are forced apart. Opening and closing may be done manually, but frequently is also accomplished by a slider which slides along the top of the bag forcing the interlocking profiles together when slid in one direction to close the bag, and having a separating means for separating the profiles when slid in the other direction to open the bag. To form bags of different depths, the diameter of the extruded tubing is changed and by methods heretofore available, the size of the fastener profiles have also been changed. This creates disadvantages in that with change in bag sizes, the size of the slider mounted at the top of the bag must also be changed. This is undesirable, as this type of product requires a very inexpensive slider which, therefore, must be mass produced in large volume. The requirement of several size sliders would make this type of slider bag economically unfeasible. Also, there frequently is a functionally optimum size for the rib and groove profiles and where change in bag size necessitated change in profile size, this results in a non-optimum size fastener. The fasteners which are too large create wrinkles in the film, require more time to cool and hence slow down the speed of production, tend to create problems at the bag machine cross seal and waste plastic, and the fasteners which are too small are either difficult to interlock or do not interlock properly and release and do not have the strength needed.
In the present invention, the tubes for forming the bags are manufactured by the extrusion of plastic through a die having an annular shaped opening with profile shaped notches to form the rib and groove profiles on the surface of the tubing. The tubing is expanded by inflation with a gas such as air as it emerges from the annular slot, and the size of the final tubing size and gauge will be controlled by the amount of air delivered inside the tube and the deflation or inflation resulting therefrom. With larger tubing for larger bags, it is necessary to increase the rate of plastic flow to the die unless the size of wall thickness is to be reduced. By controlling the rate of plastic flow, the wall thickness is controlled. The inflation of the tube combined with the axial flow of the plastic, controls the grain orientation thereof and the strength of the tube.
By the formation of tubes of different diameters, bags of different depths will be provided since the bags are formed by transverse cutting and bonding along the length of the tubing. Thus, by control of the extrusion speed and degree of inflation or deflation for a given thickness of plastic tubing, the depth of bags obtained from such tubing can be selectively controlled.
As inconvenience in the methods of the prior art in using the same die equipment for obtaining tubes of different diameters resided in the fact that with variation of the extrusion speed, which was necessary when the diameter of the tube to be obtained was changed, the size of the profiled rib and groove elements were also changed, but these elements were not subjected to the effects of inflation of the tube. For tubes of larger diameter the extrusion speed of plastic delivery had to be increased resulting in rib and groove profiles of larger size. The change in the cross-sectional size of the rib and groove elements presented a certain number of inconveniences. One of these inconveniences occurred when the tube was divided by bonding and transverse cutting into a number of bags. A bonding tool generally is arranged to bond a certain thickness and with change in thickness of the profile elements, it either slowed down the bonding operation or did not bond properly.
For bags made from tubing not utilizing such a slider and opened and closed through manual pressure, however, a variable profile size adjusted to the gauge of the tubing is a desirable feature, eliminating the inconvenience of having a preset profile with each tubing size and gauge.
Another inconvenience resulted in the use of closing and opening sliders wherein the same slider could not operate with profiles of different sizes. With the present invention wherein profiles of the same dimensions are obtained regardless of the size of the bag, the same slider can be used for different depths of bags.
It is accordingly an object of the present invention to provide a method and for producing bag forming profile tubes of different sizes and gauges by changing the extrusion speed and inflation or deflation, and simultaneously obtain rib and groove profiles of substantially constant size with the use of the same extruder.
In accordance with the present invention, the quantity of plastic material used for the formation of the rib and groove profiles is adjusted so as to be a function of the extrusion speed and a function of the inflation and deflation of the tubes so as to provide rib and groove elements of constant cross-sectional size. A further object of the invention is to provide a method using a die mechanism which achieves the above effects and which is capable of forming the rib and grooves from the same plastic source used for the formation of the wall of the tube. As will be further understood, in some instances principles of the invention may be employed by providing a source of plastic for the profile elements from a different source, and yet achieving the objectives of the present invention. Other advantages also accrue to the system of profile control described in this invention. These include the following.
Fewer changes in die components are required. Under the methods used heretofore it was necessary to change die components when changing gauges. This is no longer required. An overall faster speed of extrusion is made possible by the temperature of the profile extrudate being lower when the profile emerges out of the die. A better shaped profile is obtained. Since the profile extrudate is lower in temperature it can better assume the shape of the die. This also allows for a better stem formation at the juncture of the profile and the film and hence, less likelihood of profile lean or distortion.
Also, in accordance with the present invention wherein the same plastic material is used for the formation of the closure elements as for the tube wall, means are employed to regulate the quantity of plastic material used for the formation of the closure elements a considerable distance upstream from the profile discharge opening, so that regulation of the flow of plastic to the profile openings will not have the undesirable effects it did with prior art structures wherein the control was close to the rib and groove profile openings, and the results achieved with the present invention were not obtainable, i.e., it was not possible to maintain the same size profile while varying the gauge and size of the tube etc. In the present invention the extrusion head includes an annular channel which is located in advance of the annular extrusion slot and has a section that tapers from its entrance to its discharge and is used for the preformation of the wall of the tube. The plastic material for the formation of the closure profiles on the surface of the tube is conveyed separately by one or several separate conduits. The separation of the plastic for the profiles and that used for the formation of the wall of the tube takes place upstream of the die channel so that the means for regulating the quantity of material used for the formation of the profiles by regulating the flow of plastic through the conduit leading to the die exerts no adverse influence on the formation of the wall of the tube.
Generally, the profile ribs are positioned on the inner side of the tube so that means have to be provided for controlling the quantity of plastic material used for the profiles in such a manner so as to be accessible from the outside of the die. In order to accomplish this objective, the extrusion die head is attached to a base having a feed conduit connected to it originating from a heated plastic material source. The base consists of an outer housing and a central core, with the housing connected to the core by spokes and with the heated plastic material feeding the annular channel, flowing from its source, between the core and connected housing, and with the adjustment means for the conduit for the profile plastic passing through one of the spokes.
Various forms of the method may be employed in accordance with the principles of the invention as will become more apparent with the teachings thereof in connection with the disclosure of the preferred embodiment in the specification, claims and drawings, in which: